Z position for tool change

Our VMC with a Mitsubishi Meldas M64s lost its absolute position memory due to the batteries dying after 13 years. It started with a Z71, then with a Z70 ABS. ILLEGAL XYZ after replacing the batteries.

I have successfully homed the machine - so I'm not worried about that - but I am concerned about the tool change position being right.

I have a 600mm height gage, so I put a DTI on it and checked the height of the tool changer arm in relation to the top of the flange on the CAT40 tool, and there is a big difference in these positions. See the photos below:







It looks to me like the arm will crash into the tool holder!

Can someone walk me through the settings so that the tool changer grabs the tool at the correct height? Is HOME not the same thing as the tool change position?

Apologies for the sideways photos.

Veteran Bicycle said:

...... Is HOME not the same thing as the tool change position?

Click to expand...

Sometimes yes, sometimes no. Machine builder determines that.

Couple questions....


Does your machine use a toolchange macro? If so, post it.

What do you command in your program to do a toolchange?

G28 is your Reference Return position. Often called machine zero or zero return position. If your machine uses that position to do a toolchange, then with what is shown in your pictures you are going to smack the arm.

G30 is referred to as 2nd Reference Return position. It is a parameter set distance from the G28 Reference Return position. If your machine uses the G30 position for toolchanging then you will have to command G0 G30 Z0 in MDI to move to that position. Then check your arm alignment. If it is off, then you have to modify the parameter that sets the G30 position, or alternatively, adjust the G28 position by redoing your "absolute" encoder zero point setting.

Vancbiker said:

Sometimes yes, sometimes no. Machine builder determines that.

Couple questions....


Does your machine use a toolchange macro? If so, post it.

What do you command in your program to do a toolchange?

G28 is your Reference Return position. Often called machine zero or zero return position. If your machine uses that position to do a toolchange, then with what is shown in your pictures you are going to smack the arm.

G30 is referred to as 2nd Reference Return position. It is a parameter set distance from the G28 Reference Return position. If your machine uses the G30 position for toolchanging then you will have to command G0 G30 Z0 in MDI to move to that position. Then check your arm alignment. If it is off, then you have to modify the parameter that sets the G30 position, or alternatively, adjust the G28 position by redoing your "absolute" encoder zero point setting.

Click to expand...

A development:

Out of curiosity I went back and set Z0 so that the top of the flange of a toolholder is the same height as the top of the tool changer arm. Then in MDI I initiated a tool change with nothing in the spindle, and I got T10 FIN WAIT 0001 & 4. Z AXIS NOT 2nd HOME



So I think you were on to something.

I use Txx M6 to initiate a tool change in a program, and in the next line just Txx for the next tool to sit in the tool change pocket. Is that what you meant?

Vancbiker said:

G30 is referred to as 2nd Reference Return position. It is a parameter set distance from the G28 Reference Return position.

Click to expand...

Is it parameter 2052?

Looks like it's 8206:

I might be off base but I would think the safe home positions would be in the parameters, which you did not actually lose, correct? So upon replacement of the batts and successful homing, the machine should know where it is?

I guess my thoughts would be, "be damn certain you know what you need to do before you do it so you don't cause a crash here".

T12 M6

Am I out of the woods?

Veteran Bicycle said:

T12 M6

Am I out of the woods?

Click to expand...

Looks like you got it!

huleo said:

...... So upon replacement of the batts and successful homing, the machine should know where it is?

Click to expand...

The key here would be to verify that the homing procedure was done at the correct position.

Since the machine can find "home" once per each revolution of the ballscrew or servo motor one could think it is "homed" when it is not. Because the OP's machine appears to have some kind of alignment marks it would be pretty easy to verify that the "homing" was done at the correct point. Since not all machines have alignment marks there is usually a distance off a reference surface noted in the machine builders documentation to get one on the correct revolution of the ballscrew or servo motor to then do the "homing" procedure.

Okay, so with what I know now, I'm pretty sure I'm back in business.

What I know:

1) The tool change position (G30) is an offset from machine zero, AKA home (G28);
2) The offset between G28 and G30 is currently zero (on every axis) on my machine;
3) The offset between G28 and G30 does not have to be zero.

I feel confident that I have simply recreated the previous condition of having the two values the same, since parameter 8206 reflects a value of zero in each axis.

Is there a benefit to having the tool change position a different Z value than Z0?

Veteran Bicycle said:

......Is there a benefit to having the tool change position a different Z value than Z0?

Click to expand...

For most folks, no real benefit. Many machine builder choose to use the G28 reference position for the tool change while others use G30. Just another one of those things that a builder gets to choose how they think it best to run a machine.

"Umbrella" type toolchangers very often use both. G28 for the position that the umbrella can move to and from the spindle and G30 for the position where the umbrella is safe to rotate under the spindle. Of course the builder can reverse those too. This makes a handy way for a builder's ladder logic to verify axis position while performing the steps of a toolchange. The CNC provides an input to the PLC on completion of G28 and another input on completion of G30. This makes it easy for the builder's logic to prevent the umbrella from moving or rotating when the axis is not in a correct position.

On a machine like yours that uses G30 for the toolchange position, G28 can be set to provide a fixed reference dimension to a permanent machine component. As an example someone might set their G28 position so the spindle gage line is 15" to the centerline of their rotary axis. Then G30 would be adjusted to achieve proper alignment of the toolchange position.

depends on machine. i have also seen in tool change macro program it just goes to a specific Z position using G53 machine coordinates.
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maintenance guy goes to a special screen and it will show if tool change limit or position switches are on or ok. either tool change macro is changed or the limit switches are adjusted only if position is ok
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i have seen where cnc lost parameters and all programs and macros on control and when macro programs were download back on to cnc control they were older versions that needed editing
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just saying it maybe is done with parameters and maybe its done with macro program
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i have also seen where grid shift was adjusted, then tool change position was off too far. if grid shift Z value is -1400 and its accidentally put in as +1400 obviously that can be a problem

DMF_TomB said:

just saying it maybe is done with parameters and maybe its done with macro program

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The bright side to all of this is it has provided me an opportunity to learn these things. Nothing like a $h!t sandwich to provide inspiration!